The liftoff of space shuttle Atlantis and five military astronauts on a secret spy satellite mission was postponed today for 24 hours by fierce winds eight miles above the launch site.

"We were watching the weather all the way and finally scrubbed due to winds aloft," said launch director Bob Sieck.Weather officials reported the wind velocity 41,000 feet up was 104 mph, a condition that could seriously damage the wings of a shuttle passing through and cause the vehicle to break up.

Sieck said launch officials had decided to try again on Friday morning, but that a final decision on whether to proceed would be made about 9:30 p.m. tonight before the shuttle is fueled.

"We're going to take a hard look at the weather again," he said. "If it is clearly a no-go tomorrow, we don't want to exercise the launch team, the crew and the systems."

The time for a launch Friday would be sometime between 6:32 and 9:32 a.m. EST, the same as for today. The exact moment remained a secret because the mission is classified, but sources said the target was about 7 a.m.

Col. John Madura, an Air Force meteorologist, said the storm front that brought winds, rain and clouds to the area today would be offshore in the Atlantic on Friday. But in its wake, he said, there could be strong ground winds that could affect a launch.

Madura said that at 8:55 a.m., when the scrub announcement was made, the low-level clouds, rain and winds had moved away, but the high altitude winds remained above limits.

Madura said that if Friday's weather was unacceptable, the outlook for Saturday was not too good, with continued strong ground winds. "Sunday appears to be the best of the three days," he said.

The countdown, blacked out publicly for security reasons, stood at nine minutes when the scrub was called, sources reported. It had been held there for more than an hour while officials waited in vain for a significant break in the weather.

The astronauts, who had boarded the spacecraft shortly after 4 a.m. in their bulky escape suits, returned to crew quarters seven miles from the launch pad.

"They took it in stride," Sieck said of their reaction to the scrub. "We had talked about this threat ever since we picked up the count, so it came as no surprise."

The postponement was nothing new for the crew commander, Navy Cmdr. Robert L. Gibson. As commander of Columbia on the 24th mission, he had experienced a recordseven postponements and had to board the ship six times before finally taking off.

Technicians promptly began emptying the huge external fuel tank of the half-million gallons of liquid oxygen and liquid hydrogen that had been pumped aboard overnight. The fuel will be reused for the next attempt at launching.

The forecast for this morning had been ominous for days, and NASA knew it was gambling when it decided to load the fuel tank. The space agency has estimated the costs of a last-minute scrub at about $250,000.

The fickle weather at the Florida launch site has been a persistent problem, forcing numerous delays and postponements of previous shuttle launches. Weather criteria for launch have been tightened since the Challenger explosion of January 1986.